In the last decades, a strong effort has been put into research of innovative techniques in the field of physics and chemistry applied to Cultural Heritage for both analysis and conservation. Currently, many groups worldwide explore the possibility of developing equipment for the analysis and conservation of artifacts [1, 2], the main challenge being to obtain the more information possible without causing damage to the artifacts [3].
Classical techniques for diagnosis and conservation, as well as for restoration and consolidation, typically require transferring the artworks to be analyzed from a museum or an archeological site to a laboratory, or collecting micro-objects of the artworks [4]. Chemical information on the artworks, in relation for example to ceramic, bronzes, metals and/or pigments, may be obtained using surface spectroscopies, such as photoluminescence, Raman, X-ray photoelectron spectroscopy (XPS), X-ray-fluorescence (XRF), energy dispersive X-ray fluorescence (EDX) in scanning electron microscopy SEM, while morphological information may be obtained with scanning electron microscopy (SEM) [5]. A complete chemical characterization of bulk material may be obtained using more sophisticated, and therefore expensive, nuclear physical techniques such as particle-induced X-ray emission (PIXE) and particle-induced gamma-ray emission (PIGE) [6, 7].
Classical particle-induced X-ray emission (PIXE) and particle-induced gamma-ray emission (PIGE) comprises using heavy charged particles, such as protons, alfa-particles or sometimes heavy ions, to create inner-shell vacancies in the atoms of the object under analysis. As in the X-ray fluorescence spectroscopy and electron probe microanalysis, the X-rays and Gamma-rays produced by de-excitation of the vacancies can be measured by an energy-dispersive detection system, yielding a characteristic fingerprint of each chemical element present in the analysed bulk specimen. The incident charged-particle beam, typically consisting of protons with a mean energy of 1-5 MeV, is classically produced by a small Van de Graff accelerator or a compact cyclotron.
The advantage of using particle-induced X-ray emission (PIXE)—in the following only PIXE will be mentioned, but the same applies for PIGE when considering Gamma-rays—compared to other X-ray spectroscopies is that protons, as opposed to X-rays, can be focused and transported by electrostatic or electromagnetic devices and optics and thus can be transported over large distances without loss in the beam intensity (pencil scanning). As a result, the incident fluences on the objects are generally much higher in the particle-induced X-ray emission (PIXE) than in ordinary, true-excited X-ray Fluorescence (XRF).
Moreover, particle-induced X-ray emission (PIXE) allows performing analysis with variable spatial resolution, since protons can be focused and guided down to a beam diameter in the micrometer range. Also, the relative detection limits of particle-induced X-ray emission (PIXE) are typically two orders of magnitude better than in X-ray-fluorescence (XRF) and other electron spectroscopies such as energy dispersive X-ray fluorescence (EDX) or Auger.
Currently, PIXE is used for the analysis of a wide range of materials from proteins to cells and tissues, from polymers to ancient pigments and artefacts. Typically, in the classical particle-induced X-ray emission (PIXE) analysis of proteins or tissues, an incident proton beam, of a mean energy of about 2.5 MeV and beam current ranging from 10 nA to 150 nA, generates a spectrum with an X-ray count rate in the order of 800-2000 counts/seconds [8].
All above-mentioned techniques suffer limitations. For example, Raman and photoluminescence spectroscopy techniques require sophisticated spectrometers and lasers [9]; SEM and XPS require vacuum conditions; PIXE and PIGE require conventional particles using conventional particle accelerators, with beam energies typically ranging from a few keV to a maximum of a few MeV, which are typically available only in dedicated laboratories, since their operation requires particular analysis conditions, such as ultra-high vacuum conditions and strongly controlled temperatures [10].
Moreover, these techniques allow studying only the first superficial layers of the pieces, therefore limiting the analysis to the corrosive surface patina or to the decoration of the surface thereof, without yielding important information about the bulk material. Moreover, as they involve beam spot of a size generally of the order of μm2, they are only able to efficiently analyze small surfaces, which makes a complete analysis of larger surfaces very time consuming in so-called pencil-scanning analysis.
For example, PIXE and PIGE spectroscopy, performed on ones of the most relevant facilities in the field Cultural Heritage studies, such as the AGLAE [5] facility located at the French Louvre Laboratory C2RMF [11] or INFN-LABEC laboratory [12, 13] located in Florence, with a conventional accelerator producing proton energies ranging from 1 to 5 MeV and a beam current of the order of tens of pA to few nA, use spot sizes of the order of a few tens of microns, up to 500 μm, and require scanning the regions of interest using between tens and hundreds of points, each point taking about 100-9000 s of measuring time, to yield a complete information. A drawback of a long analysis time is that accumulation of the proton dose can damage the artifacts [14]. Moreover, the maximum analysis depth that can be obtained using these accelerator facilities is between 2 and 20 microns for typical biological film or bronze “cancer”, i. e. cuprite and malachite.
Finally, these classical techniques discussed hereinabove are typically not very easily tunable and adaptable, i.e. typically, tuning the energy of the accelerated beam takes at least tens of minutes, and their use is limited to only a certain field of energy range and to micrometric surface areas.
Scanning large bulk volumes is useful as it allows quickly identifying the presence of chemical elements, such as harmful elements for example, in the bulk. If needed, a more precise investigation can then be performed with a higher resolution, i.e. a smaller spot size and a more precise depth, in the considered volume in order to find harmful elements for example on the one hand. On the other hand, for example if no harmful elements could be detected on the larger volume, the scanned area may be characterized as “clean/healthy”, and the analysis can continue on another part of the artefact.
There is still a need in the art for a system and a method for spectroscopy of an object.